Conventional detersive surfactants comprise molecules having a water-solubilizing substituent (hydrophilic group) and an oleophilic substituent (hydrophobic group). Such surfactants typically comprise hydrophilic groups such as carboxylate, sulfate, sulfonate, amine oxide, polyoxyethylene, and the like, attached to an alkyl, alkenyl or alkaryl hydrophobe usually containing from about 10 to about 20 carbon atoms. Accordingly, the manufacturer of such surfactants must have access to a source of hydrophobe groups to which the desired hydrophile can be attached by chemical means. The earliest source of hydrophobe groups comprised the natural fats and oils, which were converted into soaps (i.e., carboxylate hydrophile) by saponification with base. Coconut oil and palm oil are still used to manufacture soap, as well as to manufacture the alkyl sulfate ("AS") class of surfactants. Other hydrophobes are available from petrochemicals, including alkylated benzene which is used to manufacture alkyl benzene sulfonate surfactants ("LAS").
The literature asserts that certain branched hydrophobes can be used to advantage in the manufacture of alkyl sulfate detersive surfactants; see, for example, U.S. Pat. No. 3,480,556 to deWitt, et al., Nov. 25, 1969. However, it has been determined that the beta-branched surfactants described in the '556 patent are inferior with respect to certain solubility parameters, as evidenced by their Krafft temperatures. It has further been determined that surfactants having branching towards the center of carbon chain of the hydrophobe have much lower Krafft temperatures. See: "The Aqueous Phase Behavior of Surfactants", R. G. Laughlin, Academic Press, N.Y. (1994) p. 347. Accordingly, it has now been determined that such surfactants are preferred for use especially under cool or cold water washing conditions (e.g., 20.degree. C.-5.degree. C.).
Generally, alkyl sulfates are well known to those skilled in the art of detersive surfactants. Alkyl sulfates were developed as a functional improvement over traditional soap surfactants and have been found to possess improved solubility and surfactant characteristics. Linear alkyl sulfates are the most commonly used of the alkyl sulfate surfactants and are the easiest to obtain. For example, long-chain linear alkyl sulfates, such as tallow alkyl sulfate, have been used in laundry detergents. However, these have significant cleaning performance limitations, especially with the trend to lower wash temperatures.
Also, as noted hereinbefore, the 2-alkyl or "beta" branched alkyl sulfate are known. In addition to U.S. Pat. No. 3,480,556 discussed above, more recently EP 439,316, published Jul. 31, 1991, and EP 684,300, published Nov. 29, 1995, describe these beta-branched alkyl sulfates. Other recent scientific papers in the area of branched alkyl sulfates include R. Varadaraj et al., J. Phys. Chem., Vol. 95, (1991), pp 1671-1676 which describes the surface tensions of a variety of "linear Guerbet" and "branched Guerbet"--class surfactants including alkyl sulfates. "Linear Guerbet" types are essentially "Y-shaped", with 2-position branching which is a long straight chain as in: ##STR1## wherein Z is, for example, OSO3Na. "branched Guerbet" types are likewise 2-position branched, but also have additional branching substitution, as in: ##STR2## wherein Z is, for example, OSO3Na. See also Varadaraj et al., J. Colloid and Interface Sci., Vol. 140, (1990), pp 31-34 relating to foaming data for surfactants which include C12 and C13 alkyl sulfates containing 3 and 4 methyl branches, respectively (see especially p. 32).
Known alkyl sulfates also include:
1. Primary akyl sulfates derived from alcohols made by Oxo reaction on propylene or n-butylene oligomers, for example as described in U.S. Pat. No. 5,245,072 assigned to Mobil Corp. PA0 2. Primary alkyl sulfates derived from oleic-containing lipids, for example the so-called "isostearyl" types; see EP 401,462 A, assigned to Henkel, published Dec. 12, 1990, which describes certain isostearyl alcohols and ethoxylated isostearyl alcohols and their sulfation to produce the corresponding alkyl sulfates such as sodium isostearyl sulfate. PA0 3. Primary alkyl sulfates, for example the so-called "tridecyl" types derived from oligomerizing propylene with an acid catalyst followed by Oxo reaction; PA0 4. Primary alkyl sulfates derived from "Neodol" or "Dobanol" process alcohols: these are Oxo products of linear internal olefins or are Oxo products of linear alpha-olefins. The olefins are derived by ethylene oligomerization to form alpha-olefins which are used directly or are isomerized to internal olefins and metathesized to give internal olefins of differering chain-lengths; PA0 5. Primary alkyl sulfates derived from the use of "Neodol" or "Dobanol" type catalysts on internal olefins derived from feedstocks which differ from those normally used to make "Neodol" or "Dobanol" alcohols, the internal olefins being derived from dehydrogenation of paraffins from petroleum; PA0 6. Primary alkyl sulfates derived from conventional (e.g., high-pressure, cobalt-catalyzed) Oxo reaction on internal olefins, the internal olefins being derived from dehydrogenation of paraffins from petroleum; PA0 7. Primary alkyl sulfates derived from conventional (e.g., high-pressure, cobalt-catalyzed) Oxo reaction on alpha-olefins; PA0 8. Primary alkyl sulfates derived from natural linear fatty alcohols such as those commercially available from Procter & Gamble Co.; PA0 9. Primary alkyl sulfates derived from Ziegler alcohols such as those commercially available from Albermarle; PA0 10. Primary alkyl sulfates derived from reaction of normal alcohols with a Guerbet catalyst (the function of this well-known catalyst is to dehydrogenate two moles of normal alcohol to the corresponding aldehyde, condense them in an aldol condensation, and dehydrate the product which is an alpha, beta- unsaturated aldehyde which is then hydrogenated to the 2-alkyl branched primary alcohol, all in one reaction "pot"); PA0 11. Primary alkyl sulfates derived from dimerization of isobutylene to form 2,4,4'-trimethyl-1-pentene which on Oxo reaction to the aldehyde, aldol dimerization, dehydration and reduction gives alcohols; PA0 12. Secondary alkyl sulfates derived from sulfuric acid addition to alpha- or internal-olefins; PA0 13. Primary alkyl sulfates derived from oxidation of paraffins by steps of (a) oxidizing the paraffin to form a fatty carboxylic acid; and (b) reducing the carboxylic acid to the corresponding primary alcohol; PA0 14. Secondary alkyl sulfates derived from direct oxidation of paraffins to form secondary alcohols; PA0 15. Primary or secondary alkyl sulfates derived from various plasticizer alcohols, typically by Oxo reaction on an olefin, aldol condensation, dehydration and hydrogenation (examples of suitable Oxo catalysts are the conventional Co, or more recently, Rh catalysts); and PA0 16. Primary or Secondary alkyl sulfates other than of linear primary type, for example phytol, farnesol, isolated from natural product sources.
Beyond such known alkyl sulfates, however, is a vast array of other possible alkyl sulfate compounds and mixtures whose physical properties may or may not make them useful as laundry detergent surfactants. (I)-(XI) display just some of the possible variations (the salts are depicted only as the common sodium salts). ##STR3##
These structures are also useful to illustrate terminology in this field: thus, (I) is a "linear" alkyl sulfate. (I) is also a "primary" alkyl sulfate, in contrast with (VII) which is a "secondary" alkyl sulfate. (II) is also a "primary" alkyl sulfate--but it is "branched". The branching is exclusively in the "2-position" as in the so-called "linear Guerbet" alkyl sulfates: carbon-counting by convention starts with C1, which is the carbon atom covalently attached to the sulfate moiety. (III) can be used to represent any one of a series of branched alkyl sulfates which, when e is an integer having the value 1 or greater, have only "non-2-position branching". According to conventional wisdom, at least for linear surfactant compounds, the hydrocarbon portion needs to have at least 12 carbon atoms, preferably more, to acquire good detergency. The indices a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q can, in principle, be adjusted to accommodate this need. Compound (VIII) is the alkyl sulfate derived from a naturally occurring branched alcohol, phytol. Compound (IX) is a highly branched alkyl sulfate, which can, for example, be made by sulfating an alcohol derived from dimerizing isobutylene and performing an Oxo reaction on the product. Compound (X), when q=14, is an isostearyl alkyl sulfate; another so-called "isostearyl" alkyl sulfate has the general structure (III)--such compounds can be made by sulfating an alcohol derived from a monomeric by product of the dimerization of oleic acid having 18 carbon atoms, i.e., d+e=14 in (III). Compound (XI) is a "neo" alkyl sulfate. (XII) and (XIII) are substructures depicting "vicinal" (XII) and "geminal" or "gem" (XIII) dimethyl branching, respectively. Such substructures can, in principle, occur in alkyl sulfates and other surfactants. Conventional alkyl sulfates can, moreover, be either saturated or unsaturated. Sodium oleyl sulfate, for example, is an unsaturated alkyl sulfate. Unsaturated alkyl sulfates such as oleyl sulfate can be relatively expensive and/or relatively incompatible with detergent formulations, especially those containing bleach.
In addition to the above structural variations, complex, highly branched primary alkyl sulfate mixtures having quaternary carbon atoms in the hydrophobe are producible, for example by sulfation of Oxo alcohol made via acid-catalyzed polygas reaction; moreover stereoisomerism, possible in many branched alkyl sulfates, further multiplies the number of species; and commercial alkyl sulfates can contain impurities including the corresponding alcohols, inorganic salts such as sodium sulfate, hydrocarbons, and cyclic byproducts of their synthesis.
One known material is sodium isostearyl sulfate which is a mixture of methyl and/or ethyl branches distributed along an otherwise linear alkyl backbone wherein the total number of carbons in the entire molecule are about 18. This isostearyl "mixture" is prepared in low yield from natural source feedstocks (i.e. tall oil, soy, etc.) via a process which results in branching which occurs in an uncontrolled manner, and which can vary depending upon the source of the feedstock. EP 401,462, assigned to Henkel, published Dec. 12, 1990 describes certain isostearyl alcohols and ethoxylated isostearyl alcohols and their sulfation to produce the corresponding alkyl sulfates such as "sodium isostearyl sulfate" (CAS 34481-82-8, sometimes referred to as "sodium isooctadecyl sulfate").
Again, while R. G. Laughlin in "The Aqueous Phase Behavior of Surfactants", Academic Press, N.Y. (1994) p. 347 describes the observation that as branching moves away from the 2-alkyl position towards the center of the alkyl hydrophobe there is a lowering of Krafft temperatures (for a 15% solution), such solubility observations teach nothing about the surfactancy of these compounds or their utility for incorporation into detergent compositions. In fact, both commercial practice and the published literature are equivocal on the desirability of branching in the mid-chain region. This includes the above-noted patent publications describing the beta-branched alkyl sulfates as the desired branching, as well as Finger et al., "Detergent alcohols--the effect of alcohol structure and molecular weight on surfactant properties", J. Amer. Oil Chemists' Society, Vol. 44, p. 525 (1967) or Technical Bulletin, Shell Chemical Co., SC: 364-80. These references assert, with respect to deleterious structural changes possible in alcohol sulfates that "moving a CH3 has a small effect". Data presented in a table shows a decrease in cotton detergency of 29% and a decrease in foaming of 77% relative to unbranched primary alcohol sulfate at the C13 chainlength. Moreover JP 721232 describes a detergency negative for the replacement of C11 linear primary alkyl sulfate with branched primary alkyl sulfate of unspecified branching.
In addition, K. R. Wormuth and S. Zushma, Langmuir, Vol. 7, (1991), pp 2048-2053 describes technical studies on a number of branched alkyl sulfates, especially the "branched Guerbet" type, derived from the highly branched "Exxal" alcohols made by Exxon. Phase studies establish a lipophile ranking, that is a hydrophobe ranking, as follows: highly branched.apprxeq.double tail&gt;methyl branched&gt;linear. Assertedly, branched surfactants mix oil and water less effectively than linear surfactants. The efficiency ranking is linear&gt;double tail&gt;&gt;methyl branched.apprxeq.highly branched. From these results, it is not immediately evident which direction to take in the development of further improvements in branched alkyl sulfates.
Thus, going beyond simple technical theories of how to achieve cleaning superiority of one pure surfactant compound versus another, the developer and formulator of surfactants for laundry detergents must consider a wide variety of possibilities with limited (sometimes inconsistent) information, and then strive to provide overall improvements in one or more of a whole array of criteria, including performance in the presence of complex mixtures of surfactants, trends to low wash temperatures, formulation changes including builders, enzymes and bleaches, various changes in consumer habits and practices, and the need for biodegradability. In the context provided by these preliminary remarks, the development of improved alkyl sulfates for use in laundry detergents and cleaning products is clearly a complex challenge. The present invention relates to improvements in such alkyl sulfate surfactant compositions.
As will be seen from the disclosures hereinafter, it has now unexpectedly been determined that certain relatively long-chain alkyl sulfate compositions containing mid-chain branching are preferred for use in laundry products, especially under cool or cold water washing conditions (e.g., 20.degree. C.-5.degree. C.). Preferred are the combination of two or more of these mid-chain branched primary alkyl sulfate surfactants which provide a surfactant mixture that is higher in surfactancy and has better low temperature water solubility than any single branched alkyl sulfate. The mixtures as produced comprise the mid-chain branching desirable for use in the surfactant mixtures of the present invention or the surfactant mixtures disclosed herein can be formulated by mixing the desired amounts of individual mid-chain branched surfactants. Such superior mixtures are not limited to combinations with other mid-chain branched surfactants but (preferably) they can be suitably combined with one or more other traditional detergent surfactants (e.g., other primary alkyl sulfates; linear alkyl benzene sulfonates; alkyl ethoxylated sulfates; nonionic surfactants; etc.) to provide improved surfactant systems.
These mid-chain branched surfactants are obtainable in relatively high purity making their commercialization cost effective for the formulator. Suitable product mixtures can be obtained from processes which utilize fossil-fuel sources. (The terms "derived from fossil fuels" or "fossil-fuel derived" herein are used to distinguish coal, natural gas, petroleum oil and other petrochemical derived, "synthetic" surfactants from those derived from living natural resources such as livestock or plants such as coconut palms).
One such process is designed to provide branched reaction products which are primarily (85%, or greater) alpha-olefins, and which are then converted into hydrophobes in an Oxo-reaction sequence. Such branched alpha-olefins contain from about 11 to about 18 (avg.) total carbon atoms and comprise a linear chain having an average length in the 10-18 region. The branching is predominantly mono-methyl, but some di-methyl and some ethyl branching may occur. Advantageously, such process results in little (1%, or less) geminal branching, i.e., little, if any, "quaternary" carbon substitution. Moreover, little (less than about 20%) vicinal branching occurs. Of course, some (ca. 20%) of the overall feedstock used in the subsequent Oxo-process may remain unbranched. Typically, and preferably from the standpoint of cleaning performance and biodegradability, this process provides alpha-olefins with: an average number of branches (longest chain basis) in the 0.4-2.5 range; of the branched material, there are essentially no branches on carbons 1, 2 or on the terminal (omega) carbon of the longest chain of the branched material.
Following the formation and purification of the branched-chain alpha-olefin, the feedstock is subjected to an Oxo carbonylation process. In this Oxo-step, a catalyst (e.g., conventional cobalt carbonyl) which does not move the double bond from its initial position is used. This avoids the formation of vinylidene intermediates (which ultimately yield less favorable surfactants) and allows the carbonylation to proceed at the #1 and #2 carbon atoms.
It is therefore an object of the present invention to provide mid-chain branched primary alkyl sulfate surfactants with greater than 14.5 carbon atoms useful in cleaning compositions. It is also an object of the present invention to provide mixtures of the mid-chain branched primary alkyl surfactants which are formulatable with other surfactants to provide cleaning compositions having one or more advantages, including greater surfactancy at low use temperatures, increased resistance to water hardness, greater efficacy in surfactant systems, improved removal of greasy or body soils from fabrics, improved compatibility with detergent enzymes, and the like.